This invention pertains to the field of transmitters and in particular to transmitters which are used in a battery-powered communication devices.
Battery-operated communication devices, such as cellular telephones or portable two-way radios, are extensively used for aerial transmission of communication messages to other communication devices within a communication system. Such communication devices include transmitters having modulating circuitry for communication messages as well as a power amplifier for amplifying the modulated signals before transmission through an antenna.
Frequency modulated (FM) transmitters are conventionally used in a variety of communication devices. An FM transmitter generally includes a class C power amplifier which provides a constant output power when modulated, thus drawing a constant current from its battery. The class C power amplifiers provide higher efficiency at increased distortion which, as is well known in the prior art, does not significantly affect system performance in FM applications.
However, newly developed, time division, multiplexed (TDM) communication systems utilize linear modulation techniques, such as QPSK, or Quad QAM, rather than frequency modulation. A linear modulator requires highly distortionless power amplifiers for transmission of the communication signals. Generally, in these applications, class AB type power amplifiers are used which, as a result of operating below the saturation region, are less efficient than class C type amplifiers. The AB type amplifiers provide non-constant output power which may have a variance in transmitter output ranging from 3 dB to 7 dB. Thus, current drain from the battery is also non-constant and may reach peak points when maximum power is being delivered by the power amplifier.
Generally, a battery power supply performs best at room temperature by presenting a low impedance which allows optimum current to be delivered to the communication device. However, as the temperature decreases, a higher impedance is presented which, under heavy load currents, limits the voltage that can be supplied to the communication device. The inefficient peak current drain situations encountered using class AB type power amplifiers, therefore, may degrade performance of the communication device at low temperatures and cause spurious transmission failures.
Currently, power amplifiers are protected from excessive heat by reducing the output power when the ambient temperature exceeds a predetermined level. However, this approach is aimed at protecting the power amplifier and does not address the problem of energy delivering capability of the battery at very low temperatures.
In many systems it has been recognized that it is not necessary for a communication device to continuously transmit at maximum capable power. This recognition has been the basis for concepts such as "Turbo Power," where the communication device initially operates at less than full power, then increases it when needed. However, this approach also does not consider the effect of temperature on the energy-delivering capability of the battery supply.